The present invention is an improvement of the Direct Process for the manufacture of alkylhalosilanes by the contact of powdered metallurgical grade silicon with an alkyl halide in the presence of a copper catalyst. The improvement reduces lot-to-lot variations in silicon conversion to desired dialkyldihalosilanes. The improvement comprises alloying 0.01 to nine weight percent copper with the silicon to be used in the process and separating slag. In a preferred embodiment of the present process, the silicon-copper alloy is pulverized to a powder and mixed with additional copper and other catalysts to form a contact mass reactive with alkylhalides.
The Direct Process for producing alkylhalosilanes is well-known and has been refined and modified in many ways since Rochow first set forth the manner in which one could obtain alkylhalosilanes by contacting alkylhalides with silicon at elevated temperatures. The process is used for producing virtually all commercial alkylhalosilanes in the world today.
Rochow in U.S. Pat. No. 2,380,995, issued Aug. 7, 1945, taught passing a gaseous stream of methyl chloride into a heated tube where it contacted powdered silicon at about 300.degree. C. Rochow obtained a mixture comprising 52 weight percent methyltrichlorosilane, 14.5 weight percent dimethyldichlorosilane, and lessor amounts of other silanes.
Commercially, the largest volume alkylhalosilane manufactured is dimethyldichlorosilane as this alkylhalosilane constitutes the backbone of most high volume commercial silicone products after it has been hydrolyzed and condensed to siloxane polymers. Therefore, it is to the benefit of the manufacturer to run the Direct Process to maximize the conversion of the raw materials to obtain the highest yield of dialkyldihalosilane. Thus one of the principal objectives of the instant invention is to control the Direct Process to maximize the overall yield of dialkyldihalosilanes, i.e. to cause the process to be as selective as possible in favor of dialkyldihalosilanes. A second objective of the instant invention is to maximize the overall yield from the raw materials. The more of the raw materials that are converted to silanes, the more economical is the process. A third objective of the present invention is to provide a copper catalyzed process where a portion of the catalytic copper can be provided by a low-cost copper source, for example, copper scrap. Copper used to create the silicon-copper alloy does not need to be in the powdered form, as is typically required for obtaining optimal performance of the Direct Process.
For purposes of this invention, the efficiency of converting raw materials is tracked by the amount of silicon charge that is converted to dialkyldihalosilane. When one considers that several million pounds of silanes are produced annually and consumed by the silicones commercial effort, it is obvious why small incremental increases in selectivity and raw materials conversion are important to the manufacturer of alkylhalosilanes.
Therefore, the manufacturers of silanes have set strict controls on the acceptable types and levels of impurities present in silicon used in the Direct Process. Clarke, J. Organometallic Chemistry, 376:165-222 (1989), provides a comprehensive review of the direct process for synthesis of methylchlorosilanes and the effects of impurities on the process. Despite the best efforts of silicon manufacturers to control the quality of silicon used in the direct process, considerable variation is observed in the performance of silicon provided by different silicon manufacturers and often between different lots of silicon provided by the same manufacturer. For unknown reasons some lots of silicon, which meet all of the silane manufacturer's quality criteria, still give lower conversion rates to the desired dialkyldihalosilanes.
Unexpectedly, the present inventors have found that this lot-to-lot variability in conversion to dialkyldihalosilanes experienced with metallurgical grade silicon can be reduced by alloying 0.01 to nine weight percent copper with the silicon to be used in the process and separating the slag. When this silicon-copper alloy is formed into a contact mass with an appropriate catalyst, the silicon which previously demonstrated reduced conversion to dialkyldihalosilanes performs comparable to good performing silicon.
Rochow, U.S. Pat. No. 2,380,995, issued Aug. 7, 1945, teaches that the reaction between silicon and a gaseous hydrocarbon halide is facilitated by the presence of a metallic catalyst such as copper. Furthermore, Rochow teaches that the copper may be alloyed with the silicon.
Rochow et al., U.S. Pat. No. 2.380.996, issued Aug. 7, 1945, teaches an improved method where a solid porous contact mass formed of powdered silicon and powdered copper is reacted with a hydrocarbon halide. This method, taught by Rochow, continues to be a method of choice for the commercial production of dialkyldihalosilanes, since use of the powdered materials provide for better control of the process than when a silicon-copper alloy is used.
Gilliam, U.S. Pat. No. 2,466,412, issued Apr. 5, 1949, teaches a process for preparing hydrocarbon substituted halosilanes where a hydrocarbon halide is reacted with an alloy of silicon and a metallic catalyst and the alloy is comminuted to a particular particle size of specific diameter and proportion by weight. Examples of metallic catalyst suggested by Gilliam are copper, nickel, tin, antimony, manganese, silver, and titanium.
Nitzsche, U.S. Pat. No. 2,666,776, issued Jan. 19, 1954, teaches the reaction of a hydrocarbon halide with an alloy which contains besides silicon and copper also a metal of the 5th to 8th groups of the periodic table, particularly cobalt, nickel, iron, or phosphorus. Nitzsche teaches that the alloy is preferably formed under slag or an inert gas. The finished alloy is reduced to pieces or ground into a powder and pressed into tablets. The alloy is then activated by wetting with a copper chloride solution or paste or by reacting with an acid such as hydrochloric acid.
Enk et al., U.S. Pat. No. 2,877,254, issued Mar. 10, 1959, reported that in the direct process, alloys of silicon with copper and heavy metals typically produced yields of 30 to 40 percent of the crude silane mixture as dimethyldichlorosilane. However, the yield of dimethyldichlorosilane could be improved to 60 to 70 percent if the aluminum content of the reaction mass was controlled below 0.2 weight percent. To reduce aluminum content, Enk et al. proposed treating the silicon, copper, heavy-metal alloy melt with a slag-forming substance such as a melt of magnesium silicate, or with magnesium or magnesium oxide in the presence of silicon dioxide.
Rossmy, U.S. Pat. No. 3,069,452, issued Dec. 18, 1962, teaches a use of a silicon-copper alloy as a catalyst for the reaction of powdered silicon with an alkylhalide. Rossmy teaches that the silicon-copper alloy catalyst is especially advantageous for use in a process where the catalyst and powdered silicon is sintered prior to reaction with the alkylhalide.
Despite the teachings of the cited prior art, variability in the performance of different lots of silicon in the Direct Process continues to plague the silicones industry. Surprisingly, the present inventors have found that if 0.01 to 9 weight percent copper is alloyed with silicon and slag appropriately removed, variability of performance of the silicon in the direct process is reduced. When the silicon-copper alloy is reacted with an alkylhalide in the presence of a catalytic concentration of copper, lot-to-lot variability in silicon conversion and in selectivity for dialkyldihalosilane is reduced.